This invention relates to semiconductor fabrication, and more specifically to MOSFET contact fabrication.
BCB: benzocyclobutene, an insulating polymer widely used as a photosensitive patterning material and insulator.
RDSon: the on-state drain-to-source resistance in a MOSFET.
UBM: under-bump metal, the conductive metal used to connect a source or gate contact to an external circuit via a surface solder bump.
MOSFET die layouts are designed to maximize the source contact area for optimum RDSon performance. This approach leaves just enough area for the gate pad to establish one wirebonding connection or bump connection. With the increasing trend toward wireless packaging via wafer bumping and flipchip technologies, a single gate bump contact serves the purpose. A disadvantage of single gate bump contact designs is that mechanical and thermal stress experienced by this single gate bump contact can cause disconnection. Inherent in multilayer metal stack technology, as commonly used in MOSFET contact areas, is the frequent problem of delamination of such metallization from its underlying dielectric layer due to differences between the coefficients of thermal expansion of the metallization and the dielectric when the stack is subjected to thermal stress. Such delamination of the metallization renders the parent device inoperable and unusable. Increasing the gate contact area in an attempt to guarantee robustness does not help, since the resulting contact still constitutes only a single connection. Producing more gate contacts through die redesign is a more-costly option, and consumes a larger portion of the source area.
By carefully repositioning MOSFET gate pads and increasing their number, the invention makes MOSFET gate connections more durable and reliable without requiring changes in either the leadframe design or the assembly process. The invention extends under-bump metal laterally from the gate pad metallization contact out to two or more gate pads not overlying the gate metallization. This approach minimizes the risk of delamination of the metallization due to thermal and mechanical stresses in assembly and operation. The invention""s use of more than one gate pad further reduces such failure risks. The gate pads are positioned sufficiently close to the original gate contact to permit use of the same leadframe design and assembly process as would be used for a single gate pad placed directly over the gate contact. The result is a reliable, durable MOSFET gate contact compatible with current assembly methods.